1. Field of the Invention
The present invention relates to a developer additive which is incorporated into a toner composition or a developer composition used for developing an electrostatic image in electrophotography, electrostatic recording and electrostatic printing. The present invention also relates to a toner composition and a developer composition containing the developer additive. In particular, the present invention relates to a developer additive capable of improving toner-fixabilities, such as low-temperature fixability and offset resistance, and pulverizability of the toner composition when it is incorporated into a toner composition or a developer composition for electrophotography.
2. Description of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809, conventional electrophotographic processes comprise uniformly charging a photoconductive insulating layer, exposing this layer to light, expelling the charge from the exposed part to thereby form an electric latent image, depositing a colored, charged fine powder called "toner" on the latent image to visualize the image (development step), transferring the visible image thus formed to a transfer material such as transfer paper (transfer step), and permanently fixing the image by heating, by the application of pressure or some other suitable fixing method (fixing step).
Therefore, the toner, that is, the toner composition, must have the functions required not only in the development step but also in the transfer and fixing steps of the electrophotographic process.
Recently, it is eagerly desired to provide prints with a higher quality at a higher speed on an electrophotographic copying machine, printer or the like. Various processes and apparatuses have been developed for fixing toner images in order to satisfy this demand. A process most ordinarily employed at present is the so-called hot roller fixing process wherein heat and pressure are simultaneously applied.
In high-speed copying machines or the like of the above-described fixing type, such a large amount of heat of the hot roller is lost by the image support that the heat supply will not meet the necessary requirements, since the fixing step is successively conducted many times and, as a result, the temperature of the hot roller is lowered which makes the fixing insufficient. Thus in high-speed copying machines or the like, the development of a toner composition which is fixable at a lower temperature is demanded.
Since polyester resins have an essentially high fixability, a toner composition comprising a polyester resin can be sufficiently fixed even by a non-contact fixing method as described, for example, in U.S. Patent No. 3,590,000. However, it has been difficult to use such a toner composition in the above-described hot roller fixing process, since an offset phenomenon is apt to occur.
The offset phenomenon is one wherein a molten toner composition is brought into contact with the surface of a hot roller in the hot roller fixing step whereby a part of the molten toner composition adheres to the surface of the hot roller and is then retransferred to the next image support to stain the image.
Thus in the high-speed copying machines of the hot-roller fixing type, the development of the following toner compositions are eagerly needed:
A: toner compositions having an excellent offset resistance, and PA1 B: toner compositions having an excellent low-temperature fixability.
To improve the offset resistance, it has been proposed to use a crosslinked resin (hereinafter referred to as means 1).
To improve the low-temperature fixability while satisfying the offset resistance, it has been proposed to use a resin having a soft segment introduced into the side chain (hereinafter referred to as means 2).
Further, a process wherein a low-molecular polyolefin wax is incorporated into the toner composition in order to impart to the toner composition a releasability from the fixing roller has also been employed (hereinafter referred to as means 3).
Furthermore, a process for reconciling the fixability of a toner composition to paper with the releasability of a toner composition from the fixing roller by improving the response of the toner composition to the heat of the fixing hot roller by incorporating an aromatic bisamide having a molecular weight of 500 to 1500 as a fixability-improving assistant into the toner composition or the developer composition has been proposed (hereinafter referred to as means 4) (see Japanese Patent Publication-A No. 986/1990).
However, the above-described means 1 has a problem that when only the crosslinked resin is used, the fixing temperature is so elevated that the unfixed part remains under ordinary fixing conditions to stain the image.
Although the fixability can be surely improved by the above-described means 2, the toughness of the resin is increased, thereby reducing the pulverizability of the resin which is concerned with the productivity of the toner composition, thereby reducing the productivity of the toner composition.
Although the above-described means 3 is effective in preventing offset, the addition of a large amount of the wax impairs the chargeability as a result of a reduction in the fluidity of the toner composition. On the contrary, when the wax is added in a small amount, no effect can be obtained.
Although the fixability can be improved by the above-described means 4, a lot-to-lot variation in the melt viscosity of the toner composition or the developer composition is caused, since the aromatic bisamide reduces the degree of polymerization, that is, the molecular weight, of the binder resin having a functional group such as an ester group and an amido group, as the case may be. Thus the stable production of the toner composition or the developer composition is difficult.
The present invention has been made under these circumstances, and thus an object of the present invention is to provide a developer additive capable of improving the toner-fixabilities, such as low-temperature fixability and offset resistance, and pulverizability of a toner composition, and a toner composition and a developer composition containing the developer additive.